A planar Bragg grating in silica is used to form an integrated optical refractive index sensor. The device, inherently suited to remote sensing using single mode transmission fibre, is shown to clearly detect phase transitions in a nematic liquid crystal and in water. Transitions from ordered to isotropic, gas to liquid and liquid to solid as well as the reverse transitions can all be clearly identified. The sensor also allows supercooled liquid to be easily identified, a task previously found challenging by other sensor technologies.